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Golden spiny mice, which inhabit rocky deserts and do not store food, must therefore employ physiological means to cope with periods of food shortage. Here we studied the physiological means used by golden spiny mice for conserving energy during food restriction and refeeding and the mechanism by which food consumption may influence thermoregulatory mechanisms and metabolic rate. As comparison, we studied the response to food restriction of another rocky desert rodent, Wagner's gerbil, which accumulates large seed caches. Ten out of 12 food-restricted spiny mice (resistant) were able to defend their body mass after an initial decrease, as opposed to Wagner's gerbils (n = 6). Two of the spiny mice (nonresistant) kept losing weight, and their food restriction was halted. In four resistant and two nonresistant spiny mice, we measured heart rate, body temperature, and oxygen consumption during food restriction. The resistant spiny mice significantly (P < 0.05) reduced energy expenditure and entered daily torpor. The nonresistant spiny mice did not reduce their energy expenditure. The gerbils' response to food restriction was similar to that of the nonresistant spiny mice. Resistant spiny mice leptin levels dropped significantly (n = 6, P < 0.05) after 24 h of food restriction, and continued to decrease throughout food restriction, as did body fat. During refeeding, although the golden spiny mice gained fat, leptin levels were not correlated with body mass (r(2) = 0.014). It is possible that this low correlation allows them to continue eating and accumulate fat when food is plentiful.

The temperatures of the arterial blood and the brain in black Bedouin goats were measured continuously by miniature data loggers. The animals were either euhydrated or dehydrated to 75-80% of the initial body mass by withholding water for 3-4 days during exposure to intense solar radiation. The daily blood temperature means and maxima of were significantly higher in dehydration than in euhydration, but 40 degreesC was rarely exceeded even during the hot hours of the day. Selective brain cooling occurred in euhydration, but its extent was small when blood temperature was below 39.5 degreesC. In dehydration, however, selective brain cooling was frequent and the standard response when blood temperature exceeded 39 degreesC. We believe that selective brain cooling contributes to the inhibition of evaporative heat loss, which is the primary cause of the higher blood temperature in dehydration. Rapid rehydration with cold water induced long-lasting depression of blood temperature. No evidence was found for mechanisms attenuating the subsequent decrease of brain temperature which occurred a few minutes after the uptake of cold water.

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